(4) Deletion and substitution of variable content

(1) Deletion of variables

[root@localhost scripts]# url=www.baidu.com.cn
[root@localhost scripts]# echo ${#url}     获取变量的长度
16
[root@localhost scripts]# echo ${url}       标准查看
www.baidu.com.cn
[root@localhost scripts]# echo ${url#*.}    从前往后，最短匹配
baidu.com.cn
[root@localhost scripts]# echo ${url##*.}   从前往后，最长匹配，贪婪匹配
cn

[root@localhost scripts]# url=www.baidu.com.cn
[root@localhost scripts]# echo ${url%.*}        从后往前，最短匹配
www.baidu.com
[root@localhost scripts]# echo ${url%%.*}       从后往前，最长匹配，贪婪匹配
www

(2) Variable index slice

[root@localhost scripts]# url=www.baidu.com.cn
[root@localhost scripts]# echo ${url:0:5}
www.b
[root@localhost scripts]# echo ${url:5:5}
aidu.
[root@localhost scripts]# echo ${url:5}
aidu.com.cn

(3) Replacement of variable content

[root@localhost scripts]# url=www.baidu.com.cn
[root@localhost scripts]# echo ${url/baidu/sina}
www.sina.com.cn
[root@localhost scripts]# echo ${url//w/N}      贪婪匹配
NNN.baidu.com.cn

(4) Substitution of variables

${变量名-新的变量值}
变量没有被赋值：会使用"新的变量值"替代
变量有被赋值（包括空值）：不会被替代

[root@localhost scripts]# unset var1
[root@localhost scripts]# echo ${var1-aaaa}
aaaa

[root@localhost scripts]# var1=aaaa
[root@localhost scripts]# echo ${var1-bbbb}
aaaa

[root@localhost scripts]# var1=
[root@localhost scripts]# echo ${var1-bbbb}